Proton conductor film, manufacturing method therefor, fuel cell provided with proton conductor film and manufacturing method therefor

ABSTRACT

A proton conductor and film thereof, electrochemical device, such as a fuel cell, employing same and methods of manufacturing same are provided. The proton conductor material film includes a proton conductor and polyvinyl alcohol as a binder for the proton conductor. The proton conductor film develops a high output by an electrode reaction and has superior hydrogen gas intercepting performance.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent Document Nos.P2001-010993 filed on Jan. 19, 2001; P2001-011114 filed on Jan. 19,2001; and P2001-010987 filed on Jan. 19, 2001, the disclosures of whichare herein incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a proton conductor film, manufacturingmethod thereof, a fuel cell provided with the proton conductor film, andmanufacturing method thereof.

As a high molecular solid electrolyte type fuel cell for automobiledriving, there has recently been known such a cell employing a proton(hydrogen ion) conducting high molecular or polymer material, such asperfluorosulfonic acid resin, e.g., a product manufactured by DuPontunder the trade name of Nafion®.

Among newer types of the known proton conductors, there arepolymolybdenic acids or oxides having a large quantity of hydratedwater, such as H₃Mo₁₂PO₄₀.29H₂O or Sb₂O₅.5.4H₂O.

When placed in a wetted state, the above-mentioned polymer materials orhydrated compounds exhibit high protonic conductivity at or near theambient temperature.

Taking an example of the perfluorosulfonic acid resin, protons desorbedfrom its sulfonic acid groups by electrical dissociation are bonded withthe moisture taken in large quantities into the polymer matrix byhydrogen linkage to yield protonated water, that is oxonium ions (H₃O⁺).The protons then are able to migrate smoothly in the polymer matrix inthe form of oxonium ions. Thus, the matrix material of this typeexhibits a rather high proton conduction effect even at an ambienttemperature. Recently, a proton conductor having a conduction mechanismentirely different from that of the above compounds has recently beendeveloped.

In this regard, a composite metal oxide of a perovskite structure, suchas SrCeO₃, doped with Yb, has been found to exhibit protonicconductivity, without using the moisture as a movement medium. In thiscomposite metal oxide, it is premeditated that the protons are conductedby channeling, by themselves, through oxygen ions forming the skeletonof the perovskite structure.

These conductive protons are not present from the outset in thecomposite metal oxide. It may be premeditated that, when this perovskitestructure contacts with steam contained in the ambient atmospheric gas,water molecules thereof at a higher temperature react with defectiveoxygen formed in the perovskite structure on doping to yield protons forthe first time by this reaction.

With the above-described various proton conductors, a number of problemsexist.

In this regard, a matrix material, such as perfluorosulfonic acid resin,must be continuously placed in a sufficiently wetted state, during use,in order to maintain a high proton conductivity. For example, theconventional proton conductor has a deficiency that its atmospheredependency is high, such that moisture or steam needs to be supplied,and moreover the operating temperature is excessively high or of anarrow range.

In a system structure of for example a fuel cell, a humidifier or othervarious ancillary devices are required, thus possibly leading to anincreased scale of the system and to increased cost in constructing thesystem. Moreover, the operating temperature range is not that wide inorder to prevent freezing or boiling of the moisture contained in thematrix.

In addition, in the case of the aforementioned composite metal oxide,having the perovskite structure, the operating temperature needs to bemaintained at as high as 500° C. or higher in order to achievemeaningful proton conduction.

A need therefore exists to provide improved proton conductors, fuelcells and methods of manufacturing same.

SUMMARY OF THE INVENTION

The present invention provides a proton conductor which is low intemperature dependency, such that it may be used in a wide temperaturerange inclusive of the ambient temperature, the lower limit temperatureis not that high, and moreover moisture is not required regardless ofwhether or not the proton conductor is used in a mobile medium, andwhich also has film forming properties and a high strength as well as agas transmission prohibiting performance.

A proton conductor according to an embodiment of the present inventionincludes a fullerene derivative including a fullerene molecule and aproton dissociative group introduced to at least one carbon atom of thefullerene molecule and a polyvinyl alcohol in an amount of more thanabout 20 wt %.

Since the proton conductor of the present invention includes thefullerene derivative and a polyvinyl alcohol in an amount of more thanabout 20 wt %, protons can be readily dissociated even under a drycondition. Moreover, since the protons are able to exhibit highconductivity over a broad temperature range (at least a range from about160° C. to a lower use limit temperature of about −40° C. which is nottoo high in distinction from the case of the conventional complex metaloxide), while exhibiting film forming properties due to the amount ofpolyvinyl alcohol, the proton conductor of the present invention isincreased in strength and prohibits gas transmission so that it may beused as a thin film exhibiting high protonic conductivity.

A method for producing a proton conductor according to an embodiment ofthe present invention includes a step of introducing proton dissociativegroups to carbon atoms of fullerene molecules to generate a fullerenederivative, and a step of mixing the fullerene derivative with polyvinylalcohol in more than of about 20 wt % and forming the resulting mixtureto a thin film.

Since the method for producing a proton conductor according to anembodiment of the present invention includes the step of generating thefullerene derivative and the step of mixing the fullerene derivativewith polyvinyl alcohol in more than of 20 wt % and forming the resultingmixture to a thin film, the proton conductor, having the specificproperties as described above, can be prepared efficiently as thin film.

An electro-chemical device according to an embodiment of the presentinvention includes a first electrode, a second electrode and a protonconductor between the first and second electrodes. The proton conductorincludes a fullerene derivative including a fullerene molecule and aproton dissociative groups introduced to at least one carbon atom of thefullerene molecule and a polyvinyl alcohol in an amount of more thanabout 20 wt %.

In the electro-chemical device according to the present invention, inwhich the proton conductor between the first and second electrodesincludes the fullerene derivative and polyvinyl alcohol in an amount ofmore than about 20 wt %, no humidifying device is required, in contrastto the conventional fuel cell having for example water as a medium formigration. Moreover, the system may be reduced in size and simplified instructure by the thin film of the proton conductor having a highstrength and superior gas impermeability.

A proton conductor film according to an embodiment of the presentinvention, containing the proton conductor and polyvinyl alcohol in anamount of more than about 20 wt %, as its binder, is heated at atemperature of from about 150° C. to about 200° C. This gives a protonconductor film improved in water-proofness and hydrogen gas interceptingperformance in which there is no risk of dissolution even on generationof water as a result of the electrode reaction.

The reason why the heating of the proton conductor film, containing theproton conductor and polyvinyl alcohol in an amount of more than about20 wt %, as its binder, at a temperature of from about 150° C. to about200° C., is not quite clear. It may be surmised that such heatingstrengthens the hydrogen bond between a number of hydroxy groupscontained in the polyvinyl alcohol to improve crystallinity of polyvinylalcohol. Apart from the precise grounds, it has been recognized thatsuch heating appreciably lowers the solubility of polyvinyl alcohol inwater while appreciably improving water-proofness of the protonconductor film employing polyvinyl alcohol as its binder.

According to the present invention, in which the heating of the protonconductor film, containing the proton conductor and polyvinyl alcohol asits binder, this leads to appreciably lowered solubility of polyvinylalcohol with respect to water, such a proton conductor film may beobtained in which there is no risk of dissolution even on watergeneration due to the electrode reaction and which has superior hydrogengas intercepting performance.

A method for producing a proton conductor film according to anembodiment of the present invention includes heating a proton conductorfilm at a temperature of about 150° C. to about 200° C., the protonconductor film includes a proton conductor admixed with polyvinylalcohol and being formed into a film.

According to the present invention, in which the proton conductor filmcontaining the proton conductor and polyvinyl alcohol as a binder forthe proton conductor is heated, and the polyvinyl alcohol is loweredappreciably in solubility with respect to water, such a proton conductorfilm can be produced in which there is no risk of dissolution even onwater generation due to the electrode reaction and which has superiorhydrogen gas intercepting performance.

A fuel cell according to an embodiment of the present invention includesa hydrogen electrode, an oxygen electrode, and a proton conductor andpolyvinyl alcohol, as a binder for the proton conductor, sandwichedbetween the hydrogen electrode and the oxygen electrode. There isprovided a proton conductor film which is heated at a temperatureranging from about 150° C. to about 200° C.

According to the present invention, in which the proton conductor filmcontaining the proton conductor and polyvinyl alcohol as a binder forthe proton conductor is heated, and in which the polyvinyl alcohol islowered appreciably in solubility with respect to water, there may beprovided a high output fuel cell which has a proton conductor film inwhich there is no risk of dissolution even on water generation due tothe electrode reaction and which has superior hydrogen gas interceptingperformance.

A method for producing fuel cell according to an embodiment of thepresent invention includes sandwiching a proton conductor film between ahydrogen electrode and an oxygen electrode, the proton conductor filmcontaining a proton conductor and polyvinyl alcohol as a binder for theproton conductor. The method for producing fuel cell includes a step ofheating the proton conductor film at a temperature of about 150° C. toabout 200° C.

According to the present invention, in which the proton conductor filmcontaining the proton conductor and polyvinyl alcohol as a binder forthe proton conductor is heated, and in which the polyvinyl alcohol islowered appreciably in solubility with respect to water, a high outputfuel cell having a proton conductor film may be provided, in which thereis no risk of dissolution even on water generation due to the electrodereaction and which has superior hydrogen gas intercepting performance.

A proton conductor film according to an embodiment of the presentinvention includes a layer of a proton conductor containing a fullerenederivative and a hydrogen gas intercepting layer composed of a fullerenederivative admixed with polyvinyl alcohol.

According to the present invention, since the hydrogen gas may bereliably prohibited from reaching the oxygen electrode, it is possibleto prevent the hydrogen gas from reaching the oxygen electrode to impedethe electrode reaction on the oxygen electrode to lower the fuel celloutput.

A fuel cell according to an embodiment of the present invention includeshydrogen electrode, an oxygen electrode, and a proton conductor filmbetween the hydrogen electrode and the oxygen electrode. The protonconductor film includes a proton conductor layer containing a fullerenederivative and a hydrogen gas intercepting layer composed of thefullerene derivative admixed with polyvinyl alcohol.

According to the present invention, it is possible to positivelyprohibit the hydrogen gas from reaching the oxygen electrode, so that itis possible to effectively prevent the hydrogen gas from reaching theoxygen electrode to impede the electrode reaction on the oxygenelectrode to lower the fuel cell output.

According to the present invention, any optional material having theability of proton conduction can be used as a proton conductor. However,from the perspective of not necessitating humidification, fullerenederivatives, such as fullerenol, are preferably employed.

As used herein, the term “fullerene derivatives” and/or other like termsmeans carbon atoms of fullerene molecules, to which the protondissociative groups are introduced.

As used herein, the term “proton dissociation” and/or other like termsmeans proton desorption due to electrical dissociation, while the term“proton dissociative groups” and/or other like terms denotes groups fromwhich protons may be desorbed on electrical dissociation.

According to the present invention, there is no particular limitation tothe fullerene molecules, to which the proton dissociative groups areintroduced, it being sufficient if the fullerene molecules, to which theproton dissociative groups are introduced, are spheroidal carbon clustermolecules. However, fullerene molecules, such as C₃₆, C₆₀, C₇₀, C₇₆,C₇₈, C₈₀, C₈₂, C₈₄ and the like, are preferably used either singly or asa mixture.

According to the present invention, the proton dissociative groups in anembodiment are those preferably represented by —XH, where X is anybivalent atom or atom group thereof and wherein H is a hydrogen atom.

According to the present invention, the proton dissociative groups in anembodiment are those preferably represented by —OH or —YOH, where Y is abivalent atom or atom group thereof and wherein H is a hydrogen atom.

According to the present invention, the proton dissociative groups in anembodiment include, for example, —OH, —OSO₃H, —COOH, —SO₃H and —OPO(OH)₂and/or the like. Typical of the fullerene derivatives are fullerenepolyhydroxide and fullerenol in the form of a hydrogen sulfate ester.

According to the present invention, the fullerene derivatives in anembodiment are preferably those in which electrophilic groups areintroduced, along with proton dissociative groups, into carbon atoms offullerene molecules thereof. The electrophilic groups include, forexample a nitro group, a carbonyl group, a carboxylic group, a nitrilegroup, a halogenated alkyl group and halogen atoms, such as fluorine orchlorine atoms, the like and combinations thereof.

According to the present invention, the number of the protondissociative groups introduced to the carbon atoms of the fullerenemolecule may be optionally determined within the range of the number ofthe carbon atoms of the fullerene molecule. It is, however, preferablyfive or more in an embodiment. In order to leave π-electricity offullerene intact to manifest effective electrophilicity, the number ofthe proton dissociative groups is desirably not larger than one half thenumber of the carbon atoms of the fullerene molecule.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B illustrate the structure of fullerene moleculesaccording to an embodiment of the present invention.

FIGS. 2A and 2B illustrate the structure of fullerene polyhydroxideaccording to an embodiment of the present invention.

FIGS. 3A to 3C are schematic views showing another example of afullerene derivative according to an embodiment of the presentinvention.

FIG. 4 is a schematic view showing an example of a proton conductoraccording to an embodiment of the present invention.

FIG. 5 illustrates a structure of a fuel cell according to an embodimentof the present invention.

FIG. 6 illustrates a structure of a hydrogen-air cell according toanother embodiment of the present invention.

FIG. 7 illustrates a schematic structure of an electro-chemical deviceaccording to still another embodiment of the present invention.

FIG. 8 illustrates a schematic structure of an electro-chemical deviceaccording to a further embodiment of the present invention.

FIGS. 9A and 9B show equivalent circuits of the fuel cell according toan embodiment of the present invention.

FIG. 10 shows the results of measurement of the complex impedance of apellet used in a fuel cell according to an embodiment of the presentinvention.

FIG. 11 shows the temperature dependency of the proton conductivity ofthe pellet used in a fuel cell according to an embodiment of the presentinvention.

FIG. 12 shows the results of measurement of the complex impedance of apellet used in a fuel cell according to an embodiment of the presentinvention.

FIG. 13 shows temperature dependency of the proton conductivity of thepellet shown in FIG. 12.

FIG. 14 is a graph showing output characteristics of the fuel cell incase of changing the ratio of the polyvinyl alcohol amount of the pelletshown in FIG. 12.

FIG. 15 is a graph showing the output against the voltage, and showingfullerenol dependency and polyvinyl alcohol dependency of the output ofthe fuel cell according to an embodiment of the present invention.

FIG. 16 is a graph showing the voltage as plotted against the currentdensity, and showing fullerenol dependency and polyvinyl alcoholdependency of the output of the fuel cell according to an embodiment ofthe present invention.

FIG. 17 is a graph showing test results of a water-proofing test in theExample 1 of the present invention.

FIG. 18 is a graph showing time changes of the hydrogen gasconcentration in the Example 2 and in the Comparative Example of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to proton conductors, filmsthereof, electro-chemical devices, such as fuel cells, that employ sameand methods of manufacturing same. The present invention is nowexplained with reference to certain preferred embodiments thereof.

In a proton conductor, a manufacturing method therefor and anelectrochemical device, according to the present invention, anyspheroidal cluster molecules may be used, without limitations, asfullerene molecules, operating as a matrix into which the protondissociative groups are introduced. In an embodiment, fullerenemolecules C₃₆, C₆₀ (see FIG. 1A), C₇₀ (see FIG. 1B), C₇₆, C₇₈, C₈₀, C₈₂and C₈₄, used either singly or in combination.

These fullerene molecules were found in 1985 in a mass analysis spectrumof a cluster beam by laser ablation of carbon (Kroto, H. W.; Heath, J.R.; O'Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985.318, 162).The manufacturing method was actually established five years later or in1990 when the manufacturing method by an arc discharge method of acarbon electrode was found. Since that time, fullerene attractedattention as a carbonaceous semiconductor material.

The present inventors have conducted research related to protonconductivity of the derivatives of the fullerene molecules and foundthat fullerene polyhydroxide, obtained on introducing hydroxy groupsinto constituent carbon atoms of fullerene, shows a high protonconductivity over a wide temperature range on both sides of the ambienttemperature, that is a temperature range including water freezing pointor a water boiling point, at least a temperature range from about 160°C. to about −40° C., even under a dry condition. It was also found thatthis proton conductivity becomes more pronounced when hydrogen sulfate(ester) groups are introduced in place of hydroxy groups into theconstituent carbon atoms of fullerene.

More specifically, fullerene polyhydroxide is a general term thatdescribes a compound composed of fullerene and a number of hydroxygroups bonded thereto, as shown in FIGS. 2A and 2B, and is commonlytermed “fullerenol”. As a matter of course, several variations arepossible as to the number of hydroxy groups or the disposition thereofin the molecule. As for this fullerenol, a first synthesis example wasreported by Chiang et al. In 1992 (Chiang, L. Y.; Swirczewski, J. W.;Hsu, C. S.; Chowdhurry, S. K.; Cameron, S.; Creegan, K. J. Chem. Soc.Chem. Commun, 1992, 1791). Since that time, fullerenol into which are aquantity in excess of a certain quantity of hydroxy groups has stirredup notice in particular as to its being water-soluble, and investigatedmainly in the bio-related technical field.

The present inventors rendered such fullerenol a flocculated product, inorder to produce interaction between hydroxy groups of fullerenemolecules (indicated by ◯ in FIG. 3) neighboring to each other, asschematically shown in FIG. 3A, and was the first to find that thisflocculated product exhibit high proton conductivity characteristics, inother words, high desorbing properties of H⁺ from the phenolic hydroxygroups of the fullerene molecules, as a macroscopic aggregate.

In an embodiment, the present invention can include, as a protonconductor, the flocculated fullerene having a number of —OSO₃H groups,in addition to fullerenol. Reports on fullerene polyhydroxide, in whichthe —OSO₃H groups are substituted for OH groups, as shown in FIG. 3B,that is hydrogen sulfate fullerenol (ester), were similarly made byChiang et al. In 1994 (Chiang L. Y.; Wang, L. Y.; Swirczewski, J. W.;Soled, S.; Cameron, S. J. Org. Chem. 1994, 59, 3960). In a certainhydrogen sulfate fullerene (ester) molecule, only a number of —OSO₃Hgroups may be contained in one molecule, or a number of these groups anda plural number of hydroxy groups may be contained in the fullerenemolecule.

As for the protonic conductivity exhibited by these derivatives as abulk material when a large number of the aforementioned fullerenederivatives are flocculated together, the large quantity of hydroxygroups, contained from the outset in the molecules, and protons derivedfrom the —OSO₃H groups, directly take part in migration, so that it isunnecessary to take hydrogen or protons, derived from steam molecules,from atmosphere, or to have the moisture replenished or absorbed fromoutside, in particular from outside air. That is, there are nolimitations on the atmosphere. It may be contemplated that fullerene asthe matric material of these derivative molecules exhibits electrophilicproperties, which possibly contributes appreciably to promotion ofelectrical dissociation of hydrogen ions in e.g., the hydroxy groups. Inthis regard, the proton conductors of the present invention are believedto exhibit superior protonic conductivity.

Moreover, since a rather large number of hydroxy groups and OSO₃H groupscan be introduced into one fullerene molecule, the number density perunit volume of the proton conductor taking part in conduction isextremely large. This is also believed to contribute to an effectiveconduction ratio displayed by the proton conductor of the presentinvention.

A substantial amount of the proton conductor of the present invention inan embodiment include fullerene carbon molecules, so that the protonconductor is lightweight and insusceptible to transmutation, while beingfree of pollutants. Moreover, the production cost of fullerene is beinglowered acutely. In the perspective of resources, environment oreconomy, fullerene can be regarded as a near-ideal carbonaceous materialas compared to any other materials.

The present inventors have recognized that the proton dissociativegroups need not necessarily be the aforementioned hydroxy groups or to—OSO₃H groups.

In this regard, the dissociative groups in an embodiment are representedby —XH, where X may be any optional bivalent atom or atom groups.Therefore, the dissociative groups in an embodiment are represented by—OH or —YOH, where Y may be any optional bivalent atom or atom groupsthereof.

Preferably, the proton dissociative groups can include —COOH, —SO₃H,—OPO(OH)₂, in addition to the aforementioned —OH or —OSO₃H andcombinations thereof.

According to an embodiment of the present invention, electrophilicgroups, such as nitro-, carbonyl-, carboxyl-, nitrile- or halogenatedalkyl groups, or halogen atoms, such as fluorine or chlorine atoms, thelike and combinations thereof are desirably introduced to carbon atomsof a fullerene molecule. FIG. 3C shows a fullerene molecule to which Zhas been introduced in addition to —OH, where Z is, for example, —NO₂,—CN, —F, —Cl, —COOR, —CHO, —COR, —CF₃ or —SOCF₃, where R denotes analkyl group and the like. If the electrophilic groups co-exist, protonsare more liable to be desorbed from the proton dissociative groups dueto the electrophilic properties of the electrophilic groups.

According to an embodiment of the present invention, the number of theproton dissociative groups introduced to the fullerene molecule may beany optional number within the range of the number of carbon atoms ofthe fullerene molecule, and desirably not less than five. Meanwhile, inorder to retain 7r-electron properties and to manifest efficaciouselectrophilicity of fullerene, the number of the above groups ispreferably not larger than one half the number of carbon atoms making upthe fullerene.

For synthesizing the fullerene derivative used in the proton conductorof the present invention, powders of the aforementioned fullerenemolecules may be subjected to an optional combination of knownprocessing operations, such as acid processing or hydrolysis, tointroduce desired proton dissociative groups to the constituent carbonatoms of the fullerene molecules.

According to an embodiment of the present invention, the powders of thefullerene derivative, thus obtained, may be pressure-molded to a desiredshape, such as to a pellet. Since no binder is needed in this case, itis possible to increase protonic conductivity and to achieve the lightweight of the proton conductor.

The proton conductor of the present invention can be used with advantageto a variety of electro-chemical devices. That is, in a basic structurethat includes first and second electrodes and the proton conductorsandwiched in-between these electrodes, the proton conductor of thepresent invention may be exploited with advantage as the interposedproton conductor.

Specifically, the proton conductor of the present invention may beapplied with advantage to an electro-chemical device where the firstand/or second electrodes is a gas electrode or to an electro-chemicaldevice where the first and/or second electrodes is an electrode of anactive material.

An embodiment of the present invention where the inventive protonconductor is used as a fuel cell is now explained.

The proton conduction in the fuel cell occurs as shown in the schematicview of FIG. 4. That is, a proton conduction unit 1 is sandwiched orpositioned between a first electrode (such as a hydrogen electrode) 2and a second electrode (such as an oxygen electrode) 3, with thedesorbed protons migrating from the first electrode 2 towards the secondelectrode 3 in a direction indicated by arrow in the drawing.

FIG. 5 shows a specified embodiment of a fuel cell employing a protonconductor of the present invention.

This fuel cell includes an anode (fuel electrode or hydrogen electrode)and a cathode (oxygen electrode) 3 facing each other and fitted withterminals 8, 9, respectively. On the anode and the cathode, catalysts 2a, 3 a are bonded tightly or scattered, respectively. In-between theseelectrodes is sandwiched the proton conduction unit 1. In use, hydrogenis supplied from an inlet 12 on the anode 2 so as to be discharged at anoutlet 13, which may optionally be omitted. As a fuel (H₂) 14 is passedthrough a flow duct 15, protons are produced to migrate towards thecathode 3 along with protons produced in the proton conduction unit 1,and are reacted with oxygen (air) 19 supplied via inlet 16 to a flowduct 17 so as to be moved towards the outlet 18, thus generating thedesired electromotive force.

In the fuel cell of the above structure, the protons supplied from theanode 2 are moved towards the cathode 3, as protons are dissociated inthe proton conduction unit 1. Thus, the fuel cell of the above-describedstructure features a high protonic conductivity. Since no humidifyingequipment is needed, it is possible to realize a simplified andlightweight system.

Moreover, in distinction from the proton conductor, composed only of afullerene derivative, the proton conductor of the present invention isafforded with film forming properties, innate to polyvinyl alcohol, suchthat, as compared to a proton conductor obtained on compression moldingthe powders of the aforementioned fullerene derivative, the protonconductor of the present invention may be used as a pliable protonconducting thin film (such as, about 300 μm or less in thickness) highin strength and which has a gas transmission prohibiting performance. Inaddition to polyvinyl alcohol, such a compound which prohibits protonicconductivity by, for example, a reaction with the fullerene derivativeto the least extent possible and which exhibits film-forming propertiesmay be used. However, such a compound not exhibiting electronicconductivity and exhibiting optimum stability may usually be employed.Specified examples of such compound include polyfluoroethylene,polyvinylidene fluoride the like and suitable combinations thereof.

A thin film of the proton conductor of the present invention may beformed by any suitable known film-forming means, such as extrusionmolding or the like.

For example, a hydrogen-air cell shown in FIG. 6 may be obtained byarranging a hydrogen electrode 21 and an air electrode 22 facing eachother with a proton conduction unit 20 in-between, sandwiching theresulting assembly by a Teflon plate 24 a and a Teflon plate 24 b havingnumerous openings 25, and securing the resulting product by bolts 26 a,26 b and nuts 27 a, 27 b. From the above electrodes, a hydrogenelectrode lead 28 a and an air electrode lead 28 b are derived tooutside.

An electro-chemical device, shown in FIG. 7, a proton conduction unit 34is sandwiched between an anode 31, carrying a layer of anode activematerial 30 on its inner surface, and a cathode 33 (gas electrode),carrying a gas transmission supporting member 32 on its outer surface.The proton conduction unit of the present invention is used as theproton conduction unit 34. As the anode active material, such a materialcomprised of a hydrogen occluding alloy or of a carbon material, such asfullerene, carrying a hydrogen occluding alloy, is preferred. As the gastransmission supporting member 32, a porous carbon paper sheet, forexample may be used. As the cathode 3, it is desirable to coat and forma material comprised of platinum carried by carbon powders. Meanwhile, agap between the outer end of the anode 31 and the outer end of thecathode 33 is closed by a gasket 35. In this electro-chemical device, asufficient quantity of the moisture is caused to be present on thecathode 33 to effect charging.

In the electro-chemical device, shown in FIG. 8, a proton conductor 41in the form of a thin film is sandwiched between an anode 38 having alayer of an anode active material 37 on its inner surface and a cathode40 having a layer of a cathode active material 39 on its inner surface.In an embodiment, the cathode active material is composed mainly of, forexample, nickel hydroxide. In the present electro-chemical device, thegap between the outer end of the anode 38 and the outer end of thecathode 40 is again closed by a gasket 42.

Each of the above-described electro-chemical device, the protonconduction effect can be displayed by a mechanism similar to that of theelectro-chemical device shown in FIG. 5. Moreover, since the protonconductor uses the fullerene derivative with the film-forming polyvinylalcohol, the proton conductor can be used as a thin film having improvedstrength and exhibiting low gas transmitting properties and hence isable to display superior protonic conductivity.

In the present embodiment, the conductor film includes a fullerenederivative, having carbon atoms which form fullerene molecules andproton (H⁺) dissociative groups introduced to the carbon atoms andpolyvinyl alcohols in a amount not less than about 20 wt % and notlarger than about 40 wt % in the perspective of the voltage or currentdependency of the fuel cell output, the PVA content preferably exceedsabout 20 wt % and is preferably about 25 wt %. The upper limit of thePVA content is preferably about 40% by weight, more preferably about 37wt % or about 35 wt %.

As a binder mixed into fullerenol, other high molecular or polymermaterial, such as polyfluoroethylene, may be used in addition to PVA, onthe condition that the PVA content exceeds about 20 wt %.

The proton conductor film of the present invention in an embodiment maybe formed into a sheet or other suitable shape and may also be appliedto hydrogen synthesis besides the fuel cell.

Moreover, according to an embodiment of the present invention, ahydrogen intercepting layer, that includes a fullerene derivativeadmixed with polyvinyl alcohol, is formed on at least one surface of theproton conductor layer containing the fullerene derivative.

According to an embodiment of the present invention, a hydrogenintercepting layer, that includes a fullerene derivative admixed withpolyvinyl alcohol, is formed on at least the surface towards the oxygenelectrode of the proton conductor layer containing the fullerenederivative.

According to an embodiment of the present invention, a hydrogenintercepting layer, that includes a fullerene derivative admixed withpolyvinyl alcohol, is formed on at least the surface towards thehydrogen electrode of the proton conductor layer containing thefullerene derivative.

In the present invention, the hydrogen intercepting layer, that includesthe fullerene derivative admixed with polyvinyl alcohol, is preferablyabout 0.1 μm to about 10 μm in thickness. If the thickness of thehydrogen intercepting layer, that includes the fullerene derivativeadmixed with polyvinyl alcohol, is less than about 0.1 μm, a sufficienthydrogen gas intercepting performance cannot be exhibited. If converselythe thickness of the hydrogen intercepting layer exceeds about 10 μm,the resistance value of the proton conductor film in its entirety isundesirably increased to cause the fuel cell output to be lowered.

In the present invention, the fullerene derivative to polyvinyl alcoholmixing weight ratio in the hydrogen intercepting layer, that includesthe fullerene derivative admixed with polyvinyl alcohol, depends on thetype of the fullerene derivative, and is preferably 6:4 to 9:1, in caseof using a hydrogen sulfate ester fullerenol as a fullerene derivative.

According to the present invention in an embodiment, the protonconductor film, containing a proton conductor and the polyvinyl alcohol,as its binder, needs to be heated at a temperature of about 150° C. toabout 200° C. If this processing temperature exceeds about 200° C., thepolyvinyl alcohol as the binder tends to be transmuted, whereas, if theprocessing temperature is lower than about 150° C., the water proofnessof the proton conductor film, employing polyvinyl alcohol as the binder,cannot be improved within a practical processing time.

In the present invention, the proton conductor film, containing theproton conductor and the polyvinyl alcohol, as its binder, is heated ata temperature preferably from about 160° C. to about 200° C.

In the present invention, the thickness of the proton conductor film,containing the proton conductor and the polyvinyl alcohol, as itsbinder, is preferably about 0.1 to about 20 μm, depending on the type ofthe proton conductor. If the thickness of the proton conductor film,containing the proton conductor and the polyvinyl alcohol, as itsbinder, is less than about 0.1 μm, it is not possible to achievesufficient hydrogen gas intercepting performance, whereas, if thethickness of the proton conductor film, containing the proton conductorand the polyvinyl alcohol, as its binder, exceeds about 20 μm, theresistance value of the proton conductor film in its entirety isundesirably increased to cause the fuel cell output to be lowered.

If, in the present invention, a fullerene derivative is used as a protonconductor, it is preferred that the fullerene derivative and polyvinylalcohol are mixed at a weight ratio of 60:40 to 95:5, depending on thetype of the fullerene derivative, to form the film of the fullerenederivative.

Although there is no particular limitation to the method of forming theproton conductor film, it may be formed by any of a variety of coatingmethods, such as a bar coating method, a spin coating method or a doctorblade method, a variety of printing methods, such as screen printing orgravure printing, or a spray-drying method.

If the printing method is used, and the fullerene derivative is used asthe proton conductor, a mixture of the fullerene derivative andpolyvinyl alcohol is dispersed or dissolved in an amount by weight ofwater equal to one to ten times as much as that of the mixture of thefullerene derivative and polyvinyl alcohol and printed. After printing,the water, as solvent, is vaporized off to form a proton conductor film.If the organic solvent is used, an organic solvent, such as methanol,ethanol or isopropyl alcohol, or a mixed solvent, composed of theseorganic solvents and water, is preferably used. After printing, thesolvent is vaporized off to form the proton conductor film.

In an embodiment of the present invention, a layer of a proton conductornot containing polyvinyl alcohol is formed on both sides of the protonconductor film.

In a preferred embodiment of the present invention, a layer of theproton conductor not containing polyvinyl alcohol is formed on bothsides of the proton conductor film, and is sandwiched by an oxygenelectrode and a hydrogen electrode, composed mainly of carbon, to form afuel cell.

While polyvinyl alcohol is water-soluble, the oxygen electrode and thehydrogen electrode, mainly composed of carbon, are water-repellent.Thus, if the proton conductor film, employing polyvinyl alcohol as abinder, is to be directly tightly contacted with the hydrogen and oxygenelectrodes, contact tightness is only poor, such that the fuel celloutput is likely to be lowered. In a preferred embodiment of the presentinvention, in which the layers of the proton conductor not containingthe polyvinyl alcohol are formed on both surfaces of the protonconductor film, contact tightness between the hydrogen and oxygenelectrodes and the proton conductor film can be improved appreciably,thus realizing a high output fuel cell.

The layer of the proton conductor not containing polyvinyl alcohol ispreferably about 5 μm to about 20 μm in thickness. If the thickness ofthe layer of the fullerene derivative not containing polyvinyl alcoholis less than about 5 μm, contact tightness with the hydrogen and oxygenelectrodes, composed mainly of carbon, cannot be improved sufficiently,whereas, if the thickness of the layer of the proton conductor notcontaining polyvinyl alcohol exceeds about 20 μm, the resistance valueof the proton conductor film in its entirety is undesirably increased tocause the fuel cell output to be lowered.

Although there is no particular limitation to the method of forming theproton conductor film, not containing polyvinyl alcohol, it may beformed by any of a variety of coating methods, such as a bar coatingmethod, a spin coating method or a doctor blade method, a variety ofprinting methods, such as screen printing or gravure printing, or aspray-drying method. If the printing method is used, the fullerenederivative is dispersed or dissolved in an amount of an organic solvent,such as tetrahydrofuran, acetonitrile, dimethylacetoamide,dimethylformamide or N-methylpyrrolidone, equal to one to ten times byweight as much as that of the fullerene derivative, and printed. Afterprinting, the solvent is vaporized off to form a fullerene derivativelayer not containing polyvinyl alcohol.

According to an embodiment of the present invention, the protonconductor film contains a fullerene derivative and polyvinyl alcohol.There is also provided a hydrogen gas intercepting layer that includesthe fullerene derivative admixed with polyvinyl alcohol.

According to an embodiment of the present invention, it has beenconfirmed that, by providing the hydrogen gas intercepting layercomposed of the fullerene derivative admixed with polyvinyl alcohol, tothe proton conductor film, the hydrogen gas intercepting performance ofthe proton conductor film can be improved appreciably.

According to an embodiment of the present invention, it is sufficient ifthere is provided a hydrogen gas intercepting layer composed of thefullerene derivative admixed with polyvinyl alcohol, irrespective of theposition for forming the hydrogen gas intercepting layer.

According to an embodiment of the present invention, the hydrogen gasintercepting layer, composed of the fullerene derivative admixed withpolyvinyl alcohol, is formed on at least one surface of the protonconductor film containing the fullerene derivative. It should be notedthat the hydrogen gas intercepting layer, composed of the fullerenederivative admixed with polyvinyl alcohol, may be formed on the surfacetowards the oxygen electrode of the proton conductor film or on thesurface towards the hydrogen electrode of the proton conductor film.

According to an embodiment of the present invention, the hydrogen gasintercepting layer is formed to a thickness of about 0.1 to about 10 μm.If the thickness of the hydrogen gas intercepting layer, composed of thefullerene derivative admixed with polyvinyl alcohol, is less than about0.1 μm, it is not possible to realize sufficient hydrogen gasintercepting performance, whereas, if the thickness exceeds about 10 μm,the resistance value of the proton conductor film in its entirety isundesirably increased to cause the fuel cell output to be lowered. Themixing ratio by weight of the fullerene derivative to polyvinyl alcoholin the hydrogen gas intercepting layer, composed of the fullerenederivative admixed with polyvinyl alcohol, is preferably 6:4 to 9:1, incase of employing hydrogen sulfate ester furalenol as the fullerenederivative. This mixing ratio is varied depending on the type of thefullerene derivative used.

Although there is no particular limitation to the method for forming thehydrogen gas intercepting layer, composed of the fullerene derivativeadmixed with polyvinyl alcohol, it may be formed by a variety of coatingmethods, such as a bar coating method, a spin coating method or a doctorblade method, a variety of printing methods, such as screen printing orgravure printing, or a spray-drying method. If the printing method isused, the mixture of the fullerene derivative and polyvinyl alcohol isdispersed or dissolved in an amount of water equal to one to ten timesby weight as much as that of the mixture of the fullerene derivative andpolyvinyl alcohol, and printed. After printing, the water as solvent isvaporized off to form the hydrogen gas intercepting layer, composed ofthe fullerene derivative admixed with polyvinyl alcohol.

According to an embodiment of the present invention, a layer of theproton conductor not containing polyvinyl alcohol is formed on thesurface of the hydrogen gas intercepting layer, composed of thefullerene derivative admixed with polyvinyl alcohol.

According to an embodiment of the present invention, the heating of theproton conductor film may be carried out at any time following thedeposition of the proton conductor film.

The heating may be carried out after forming the proton conductor filmon the oxygen electrode or the hydrogen electrode, formed mainly ofcarbon. Alternatively, the heating may be carried out after forming aproton conductor film on the oxygen electrode or the hydrogen electrode,formed mainly of carbon, and tightly contacting the oxygen electrode orthe hydrogen electrode on the proton conductor film surface.

For forming the layer of the fullerene derivative not containingpolyvinyl alcohol, it is possible to form a layer of the fullerenederivative, not containing polyvinyl alcohol, on the oxygen electrode orthe hydrogen electrode, formed mainly of carbon, to form a protonconductor film thereon, and to then effect the heating. It is alsopossible to form a layer of the fullerene derivative not containingpolyvinyl alcohol on the oxygen electrode or the hydrogen electrode,formed mainly of carbon, to form a proton conductor film thereon, toform a layer of a fullerene derivative not containing polyvinyl alcoholon the surface of the proton conductor film, to contact the oxygen orhydrogen electrode tightly with the layer of the fullerene derivativenot containing polyvinyl alcohol, and to then effect the heating.

For providing the hydrogen intercepting layer, admixed with polyvinylalcohol, on the layer of the fullerene derivative, the hydrogenintercepting layer is first formed to then effect the heating.

It is also possible to effect the heating solely of the proton conductorfilm as formed.

There is no particular limitation to the methods for heating the protonconductor film. For example, a method of setting a proton conductor filmor a layered product, including the proton conductor film, on a heater,and heating the proton conductor film or the layered product, a methodof directly pressuring a heater onto the proton conductor film to heatit (hot press method), a method of charging a proton conductor film or alayered product, including the proton conductor film, into a constanttemperature vessel, and heating the proton conductor film or the layeredproduct, or a method of blowing a gas, containing heated air or a heatedgas, such as heated nitrogen or argon, onto a proton conductor film or alayered product, including the proton conductor film, to effect heating,the like can be used according to an embodiment of the presentinvention.

The heating of the proton conductor film or the layered product,including the proton conductor film, may be carried out in an atmosphereof inert gases, such as argon or nitrogen, or in outside air, the likeand suitable mixtures thereof.

The present invention is now explained in detail, without limitations,with reference to Examples.

<Synthesis of Fullerene Polyhydroxide>

This synthesis was carried out by having reference to a referencematerial (Chiang, L. Y.; Wang, L. Y.; Swirczewski, J. W.; Soled, S.;Cameron, S. J. Org. Chem. 1994, 59, 3960). 2 g of powders of a C₆₀/C₇₀fullerene mixture, containing approximately 15% of C₇₀, were chargedinto 30 ml of fuming sulfuric acid and stirred for three days in anitrogen atmosphere maintained at 57° C. The resulting reaction productwas charged gradually into anhydrous diethyl ether, and cooled in aglacial bath. The resulting precipitates were fractionated oncentrifugation, washed thrice with diethyl ether and twice with a 2:1liquid mixture of diethyl ether and acetonitrile, and dried at 40° C.under reduced pressure. The resulting dried product was charged into 60ml of ion exchanged water and stirred for ten hours under bubbling withnitrogen at 85° C. The reaction product was then freed on centrifugationfrom precipitates which were further washed several times with purewater. After repeated centrifugation, the resulting product was dried at40° C. under reduced pressure. The resulting brownish powders weresubjected to FT-IR measurement. It was found that the IR spectrum of thebrownish powders substantially coincided with that of C₆₀(OH)₁₂indicated in the above reference material, such that the powders couldbe identified to fullerene polyhydroxide as a target substance.

<Manufacture of Flocculated Fullerene Polyhydroxide>

90 mg of these powders of fullerene polyhydroxide were taken and pressedunidirectionally to form a circular pellet 15 mm in diameter. Thepressing pressure at this time was approximately 7 ton/cm². It was foundthat these powders of fullerene polyhydroxide were superior inmoldability, despite the fact that they were completely free of e.g., abinder resin, and could be readily formed into a pellet. This pellet,approximately 300 μm in thickness, is a pellet of Example 1.

<Synthesis 1 of Fullerene Polyhydroxide Hydrogensulfate (Full Ester)>

This synthesis was conducted by having reference to the above-mentionedreference material. 1 g of powders of fullerene polyhydroxide wascharged into 60 ml of fuming sulfuric acid and stirred at roomtemperature for three days in a nitrogen atmosphere. The resultingreaction product was charged gradually into anhydrous diethyl ether andcooled in a glacial bath. The resulting precipitates were fractionatedon centrifugation, washed thrice with diethyl ether and twice with a 2:1liquid mixture of diethyl ether and acetonitrile, and dried at 40° C.under reduced pressure. The resulting powders were subjected to FT-IRmeasurement. It was found that the IR spectrum of the powderssubstantially coincided with that of fullerene polyhydroxide, allhydroxy groups of which were replaced by hydrogen sulfate, as indicatedin the above reference material, such that the powders could beidentified to be a target substance.

<Manufacture of Flocculated Pellet of Fullerene PolyhydroxideHydrogensulfate (Ester)>

70 mg of these powders of fullerene polyhydroxide hydrogensulfate(ester) were taken and pressed unidirectionally to form a circularpellet 15 mm in diameter. The pressing pressure at this time wasapproximately 7 ton/cm². It was found that these powders of fullerenepolyhydroxide were superior in moldability, and could be readily formedinto a pellet, despite the fact that they were completely free of e.g.,a binder resin. This pellet, approximately 300 μm in thickness, is apellet of Example 2.

<Synthesis 2 of Fullerene Polyhydroxide Hydrogensulfate (Partial Ester)>

2 g of powders of C₆₀/C₇₀ fullerene mixture containing approximately 15%of C₇₀ were charged into 30 ml of fuming sulfuric acid and stirred at57° C. for three days in a nitrogen atmosphere. The resulting reactionproduct was charged gradually into anhydrous diethyl ether and cooled ina glacial bath. It is noted that diethyl ether used at this time was anon-hydrated product. The resulting precipitates were fractionated oncentrifugation, washed thrice with diethyl ether and twice with a 2:1liquid mixture of diethyl ether and acetonitrile, and dried at 40° C.under reduced pressure. The resulting powders were subjected to FT-IRmeasurement. It was found that the IR spectrum of the powderssubstantially coincided with that of fullerene derivative, partiallycontaining hydroxy groups and OSO₃H groups, as indicated in the abovereference material, such that the powders could be identified to be atarget substance.

<Manufacture 2 of Flocculated Pellet of Fullerene Polyhydroxide HydrogenSulfate (Ester)>

80 mg of these powders of fullerene polyhydroxide hydrogensulfate esterwere taken and pressed unidirectionally to form a circular pellet 15 mmin diameter. The pressing pressure at this time was approximately 7ton/cm². It was found that these powders of fullerene polyhydroxide weresuperior in moldability, and could be readily formed into a pellet,despite the fact that they were completely free of e.g., a binder resin.This pellet, approximately 300 μm in thickness, is a pellet of Example3.

<Manufacture of Flocculated Fullerene Pellet>

For comparison, 90 mg of fullerene powders used as a starting materialfor synthesis in the above Examples were taken and pressedunidirectionally to form a circular pellet 16 mm in diameter. Thepressing pressure at this time was approximately 7 ton/cm². It was foundthat these powders of fullerene polyhydroxide were acceptable inmoldability, and could be formed relatively readily into a pellet,despite the fact that they were completely free of e.g., a binder resin.This pellet, approximately 300 μm in thickness, is a pellet ofComparative Example 1.

<Measurement of Protonic Conductivity of Pellets Obtained in theRespective Examples and Comparative Example>

For measuring the conductivity of the pellets of the Examples 1 to 3 andthe Comparative Example 1, each pellet was clinched on its both sides byaluminum discs 15 mm in diameter as in the pellet. An AC voltage, withan amplitude of 0.1V and a frequency ranging from 7 MHz to 0.01 Hz, wasapplied thereto for measuring the complex impedance at each frequency.The measurement was conducted in a dry atmosphere.

In measuring the impedance, the proton conduction unit 1 of the protonconductor, formed by the pellet, electrically constitutes an equivalentcircuit shown in FIG. 9A. Specifically, capacitors 6 a, 6 b are formed,along with the proton conduction unit 1 represented by a parallelcircuit of a resistor 4 and a capacitor 5. Meanwhile, the capacitor 5represents the delay effect in proton migration (phase delay in case ofa high frequency), while the resistor 4 is a parameter for protonmobility.

It should be noted that the measured impedance Z is given byZ=Re(Z)+i·Im(Z). The frequency dependency of the proton conduction unitrepresented by the above equivalent circuit was checked.

Meanwhile, FIG. 9B is an equivalent circuit for the case of employingroutine fullerene molecules not indicating proton dissociativeproperties (Comparative Example as later explained).

FIG. 10 shows the results of impedance measurement for the pellet inExample 1 and Comparative Example 1.

These results indicate that, in the Comparative Example 1, the frequencyresponse of the complex impedance is approximately the same as that of acapacitor taken alone, while no conduction behavior of charged particles(electrons or ions) of the flocculated mass of fullerene itself wasobserved, as shown at B in FIG. 10. In Example 1, an extremely neat solesemi-circular arc shape, somewhat flat, can be noticed in the highfrequency area, as shown at A in FIG. 10. This indicates that some orother conduction behavior of charged particles is present in the pellet.Moreover, in a low frequency area, there may be observed a rapid rise inthe imaginary portion of the impedance. This indicates that chargedparticles are subjected to blocking with respect to the aluminumelectrode as the DC voltage is approached gradually. As a matter ofcourse, since the charged particles on the side aluminum electrode areelectrons, the charged particles within the pellet are not electrons norholes but are other charged particles, that is ions. Judging from thestructure of the fullerenol used, these charged particles cannot beother than protons.

The conductivity of these charged particles can be found from an X-axisintercept of the arc on the high frequency side. In the pellet ofExample 1, this conductivity is calculated to be approximately 5×10⁻⁶S/cm. Similar measurements were made on the pellets of Examples 2 and 3and impedance frequency response similar in overall shape to the case ofExample 1 were obtained. However, the conductivity, as found from theX-axis intercept of the arcuate portion, was of different values, asshown in Table 1:

TABLE 1 Conductivity (25° C.) of proton conductor pellet in the presentinvention pellet types conductivity (S/cm) Ex. 1 5 × 10⁻⁶ Ex. 2 9 × 10⁻⁴Ex. 3 2 × 10⁻⁵

It may be seen that, when the OSO₃H groups are substituted for hydroxygroups, the conductivity in the pellet tends to be increased. This isdue to the fact that, with the OSO₃H groups, electrical dissociation ofhydrogen is more likely to occur than with the hydroxy groups. It couldbe found that, with the flocculated mass of this sort of the fullerenederivative, proton conduction is possible at room temperature in a dryatmosphere not only when one of the hydroxy groups and the OSO3H groupsexist singly but also when both of the hydroxy groups and the OSO3Hgroups co-exist.

Using the pellet of Example 1, the complex impedance was measured in atemperature range from 160° C. to −40° C. and temperature dependency ofthe conductivity as found from the arc on the high frequency side waschecked. FIG. 11 shows the results by an Arrhenius type plot. It may beseen from this figure that conductivity is changed acutely straightlyfor a temperature range from 160° C. to −40° C. That is, this figureindicates that the sole ionic conduction mechanism is able to proceed inthis temperature range. In short, it may be seen that a sole ionicconductivity mechanism can proceed in this temperature range. That is,with the proton conductor of an embodiment of the present invention,conduction becomes possible in a broad temperature range, in particularin an elevated temperature such as about 160° C. or a low temperaturesuch as about −40° C.

<Manufacture A of Fullerene Polyhydroxide Pellet>

75 mg of powders of fullerene polyhydroxide, obtained by theaforementioned synthesis method, were taken and mixed with 25 mg ofpolyvinyl alcohol. The resulting product was further mixed with 0.5 mlof dimethylformamide and stirred thoroughly. This mixture was caused toflow into a circular mold 15 mm in diameter and the solvent wasvaporized off under reduced pressure. The press working was thenperformed to produce a pellet 15 mm in diameter. This pellet was of athickness of approximately 300 μm. This pellet is a pellet of Example 4.

<Synthesis 1A of Fullerene Polyhydroxide Hydrogen Sulfate (Full Ester)>

This synthesis was conducted by having reference to the above-mentionedreference material. 1 g of powders of fullerene polyhydroxide wascharged into 60 ml of fuming sulfuric acid and stirred at roomtemperature for three days in a nitrogen atmosphere. The resultingreaction product was charged gradually into anhydrous diethyl ether andcooled in a glacial bath. The resulting precipitates were fractionatedon centrifugation, washed thrice with diethyl ether and twice with a 2:1liquid mixture of diethyl ether and acetonitrile and dried at 40° C.under reduced pressure. The resulting powders were subjected to FT-IRmeasurement. It was found that the IR spectrum of the powderssubstantially coincided with that of fullerene polyhydroxide, allhydroxy groups of which were replaced by hydrogen sulfate ester groups,as indicated in the above reference material, thus indicating that thepowders were a target substance.

<Manufacture 1A of Fullerene Polyhydroxide Hydrogen Sulfate EsterPellet>

75 mg of powders of fullerene polyhydroxide hydrogen sulfate (ester)were taken and mixed with 25 mg of polyvinyl alcohol. The resultingproduct was further mixed with 0.5 ml of dimethylformamide and stirredthoroughly. This mixture was caused to flow into a circular mold 15 mmin diameter and the solvent was vaporized off under reduced pressure.The press working was then performed to produce a pellet 15 mm indiameter. This pellet was of a thickness of approximately 300 μm. Thispellet is a pellet of Example 5.

<Synthesis 2A of Fullerene Polyhydroxide Hydrogen Sulfate (PartialEster)>

2 g of powders of C₆₀/C₇₀ fullerene mixture, containing approximately15% of C₇₀, were charged into 30 ml of fuming sulfuric acid and stirredat 57° C. for three days in a nitrogen atmosphere. The resultingreaction product was charged gradually into anhydrous diethyl ether andcooled in a glacial bath. It is noted that diethyl ether used at thistime was a non-hydrated product. The resulting precipitates werefractionated on centrifugation, washed thrice with diethyl ether andtwice with a 2:1 liquid mixture of diethyl ether and acetonitrile anddried at 40° C. under reduced pressure. The resulting powders weresubjected to FT-IR measurement. It was found that the IR spectrum of thepowders substantially coincided with that of fullerene derivative,partially containing hydroxy groups and OSO₃H groups, as indicated inthe above reference material, thus indicating that the powders were atarget substance.

<Manufacture 2A of Fullerene Polyhydroxide Hydrogen Sulfate EsterPellet>

75 mg of powders of fullerene polyhydroxide, partially in the form of ahydrogen sulfate ester, were taken and mixed with 25 mg of polyvinylalcohol. The resulting product was further mixed with 0.5 ml ofdimethylformamide and stirred thoroughly. This mixture was caused toflow into a circular mold 15 mm in diameter and the solvent wasvaporized off under reduced pressure. The press working was thenperformed to produce a pellet 15 mm in diameter. This pellet was of athickness of approximately 300 μm. This pellet is a pellet of Example 6.

<Manufacture of Fullerene Pellet>

For comparison, 75 mg of fullerene powders used as a starting materialfor synthesis in the above Examples, were taken and mixed with 25 mg ofpowders of polyvinyl alcohol. The resulting mixture was added to with0.5 ml of dimethylformamide and stirred thoroughly. The resultingmixture was caused to flow into a circular mold 15 mm in diameter andthe solvent was vaporized off under reduced pressure. The resulting masswas pressed to form a circular pellet 15 mm in diameter. This pellet,approximately 300 μm in thickness, is a pellet of Comparative Example 2.

<Measurement of Protonic Conductivity of Pellets Obtained in theRespective Examples and Comparative Example>

For measuring the conductivity of the pellets of the Examples 4 to 6 andthe Comparative Example 2, each pellet was clinched on its both sides byaluminum discs 15 mm in diameter as in the pellet. An AC voltage, withan amplitude of 0.1 V and a frequency ranging from 7 MHz to 0.01 Hz, wasapplied thereto for measuring the complex impedance at each frequency.The measurement was conducted in a dry atmosphere.

In measuring the impedance, the proton conduction unit 1 of the protonconductor, formed by the pellet, electrically constitutes an equivalentcircuit shown in FIG. 9A. Specifically, capacitors 6 are formed betweenthe first and second electrodes 2, 3, along with the proton conductionunit 1 represented by a resistor 4. Meanwhile, the capacitor 6 representthe delay effect in proton migration (phase delay in case of a highfrequency), while the resistor 4 is a parameter for proton mobility. Itis noted that the measured impedance Z is given by Z=Re(Z)+i·Im(Z). Thefrequency dependency of the proton conduction unit represented by theabove equivalent circuit was checked. Meanwhile, FIG. 9B is anequivalent circuit for the case of employing routine fullerene moleculesnot indicating proton dissociative properties (Comparative Example aslater explained).

FIG. 12 shows the results of impedance measurement for the pellet inExample 4 and Comparative Example 2.

These results indicate that, in the Comparative Example 2, the frequencyresponse of the complex impedance is approximately the same as that of acapacitor, taken alone, while no conduction behavior of chargedparticles (electrons or ions) of the flocculated mass of fullereneitself was observed, as shown at D in FIG. 12. In Example 4, anextremely neat sole semi-circular arc shape, somewhat flat, can benoticed in the high frequency area, as shown at C in FIG. 12. Thisindicates that some or other conduction behavior of charged particles ispresent in the pellet. Moreover, in a low frequency area, there may beobserved a rapid rise in the imaginary portion of the impedance. Thisindicates that charged particles are subjected to blocking with respectto the aluminum electrode as the DC voltage is approached gradually. Asa matter of course, since the charged particles on the side aluminumelectrode are electrons, the charged particles within the pellet are notelectrons nor holes but other charged particles, that is ions. Judgingfrom the structure of the fullerenol used, these charged particlescannot be other than protons.

The conductivity of these charged particles can be found from an X-axisintercept of the arc on the high frequency side. In the pellet ofExample 4, this conductivity is calculated to be approximately 1×10⁻⁶S/cm. Similar measurements were made on the pellets of Examples 5, 6 andon the pellets of Examples 4 to 6 and impedance frequency responsesimilar in overall shape to the case of Example 4 were obtained.However, the conductivity, as found from the X-axis intercept of thearcuate portion, was of different values, as shown in Table 2:

TABLE 2 Conductivity (25° C.) of proton conductor pellet in the presentinvention pellet types conductivity (S/cm) Ex. 4 1 × 10⁻⁶ Ex. 5 2 × 10⁻⁴Ex. 6 6 × 10⁻⁵

It may be seen that, when the OSO₃H groups are substituted for hydroxygroups, the conductivity in the pellet tends to be increased. This isdue to the fact that, with the OSO₃H groups, electrical dissociation ofhydrogen is more likely to occur than with the hydroxy groups. It couldbe found that, with the flocculated mass of this sort of the fullerenederivative, proton conduction is possible at room temperature in a dryatmosphere not only when one of the hydroxy groups and the OSO₃H groupsis present singly but also when both of the hydroxy groups and the OSO3Hgroups are present together.

Using the pellet of Example 4, the complex impedance was measured in atemperature range from 160° C. to −40° C. and temperature dependency ofthe conductivity as found from the arc on the high frequency side waschecked. FIG. 13 shows the results by an Arrhenius type plot. It may beseen from this figure that conductivity is changed acutely straightlyfor a temperature range from 160° C. to −40° C. That is, this figureindicates that the sole ionic conduction mechanism is able to proceed inthis temperature range. In short, it may be seen that a sole ionicconductivity mechanism can proceed in this temperature range. That is,with the proton conductor of the present invention, conduction becomespossible in a broad temperature range, in particular in an elevatedtemperature such as 160° C. or a low temperature such as −40° C.

<Preparation of Fuel Cell and Evaluation of its Performance>

A proton conductor was formed from a mixture of the fullerenepolyhydroxide hydrogen sulfate ester used in Example 5 and the binder(polyvinyl alcohol). The mixing ratio of the fullerene polyhydroxidehydrogen sulfate ester, referred to below as FL, and polyvinyl alcohol,referred to below as PVA {PVA/(FL+PVA)} in unit of wt %, sometimesabbreviated to %, was changed to various values, fuel cells associatedwith these variable values were constructed as shown in FIG. 8. To theanode and cathode of each fuel cell were supplied hydrogen and oxideand, as each fuel cell was run in the constant current mode,measurements were made of output characteristics of each fuel cell.

As a result, output characteristics proportionate to the PVA content inthe proton conductor, with the output peak voltage of 400 mV, wereobtained, as may be seen from FIG. 14.

Thus, it may be seen that a point of inflection is presented for the PVAcontent of about 20%, and that the output is decreased precipitously forthe PVA content less than about 20%, while a high output is obtained forthe PVA content exceeding about 20%. From these results, the PVA contentshould be of a value exceeding about 20%.

Since the point of inflection is presented at about 25%, the lower limitof the PVA content is desirably about 25%. On the other hand, the outputstarts to be decreased with about 33% as a peak point and a point ofinflection is presented at about 37%. Since the output itself is liableto be lowered significantly beyond about 40%, the upper limit of the PVAcontent is preferably about 40% and more preferably about 37%.

Thus, from the results of FIG. 14, the PVA content should exceed 20% andis preferably 25%, with the upper limit value being preferably about40%, more preferably about 37% and most preferably about 35%. The meritof this value of the PVA mixing ratio is apparent from data ofvoltage/current density dependency of the output, as may be indicatedfrom FIGS. 15 and 16, which will be explained subsequently.

FIGS. 15 and 16 indicate the FL dependency and PVA dependency,respectively, of the output for the lowest PVA content (12.5%), thehighest PVA content (50%), the PVA content indicating the peak value(33%) and the PVA content for the inflection point (25%), in the fuelcell manufactured for conducting the above measurement. FIGS. 2 and 3are graphs are a graph showing output/voltage characteristics and agraph showing voltage/current density characteristics, respectively.

That is, in the output/voltage characteristics of FIG. 15, output peakvalues occur for 400 mV. In particular, when the PVA content is 25% and33%, the output increase and decrease degrees indicate sharp symmetryfor voltage values lower and higher than 400 mV as a boundary value,with the output performance being kept up to 900 mV. Thesecharacteristics are similarly noticed in voltage/current densitycharacteristics shown in FIG. 16. Thus, it may be demonstrated that asuperior fuel cell may be realized by employing a proton conductorcontaining the fullerene derivative and polyvinyl alcohol in an amountof more than about 20 wt %.

EXAMPLE 1

A liquid dispersion was prepared by dispersing fullerenol in the form ofa hydrogen sulfate ester in tetrahydrofuran in a weight ratio of 1:2.

The so produced liquid dispersion was coated on a carbon electrode,using a mask and a squeezee, and dried to vaporize tetrahydrofuran, as asolvent, to form a layer of fullerenol in the form of a hydrogen sulfateester on a carbon electrode to a thickness of 10 μm.

Then, fullerenol in the form of a hydrogen sulfate ester and polyvinylalcohol were mixed together at a weight ratio of 2:1 and dispersed ordissolved in a quantity of water equal to five times by weight as muchas the total weight of fullerenol in the form of a hydrogen sulfateester and polyvinyl alcohol to prepare a slurry. This slurry was thencoated on the layer of fullerenol in the form of a hydrogen sulfateester, using a mask and a squeezee. Then, water was vaporized off toform a proton conductor film, 12 μm in thickness, containing fullerenolin the form of a hydrogen sulfate ester and polyvinyl alcohol.

A heater was directly applied, under pressure, to plural laminatedproducts, each comprised of carbon electrodes, a layer of fullerenol inthe form of a hydrogen sulfate ester and a layer of polyvinyl alcohol,to heat the proton conductor film with variable heating temperature andheating time.

As the variable heating temperatures, 120° C., 150° C., 160° C., 180° C.and 200° C. were selected.

The laminated products, thus heated, were immersed in water and allowedto stand for one minute to conduct tests on water-proofness, in whichthe state of dissolution of the proton conductor film into water wasobserved with naked eyes.

The test results are shown in FIG. 17, in which x, Δ and ● indicate thatthe majority of the proton conductor film was found to be dissolved,that approximately one half of the proton conductor film was found to bedissolved and that 90% or more of the proton conductor film was leftover without dissolution, respectively.

It is seen from FIG. 17 that, when the proton conductor film was heatedat 200° C., 180° C. or 160° C., water-proofness of the proton conductorfilm was improved appreciably within the processing time of 10 seconds,whereas, if the proton conductor film was heated at 150° C., thewater-proofness of the proton conductor film could not be improved in adesired manner unless heating is continued for 20 seconds or longer.

On the other hand, when the proton conductor film was heated at atemperature exceeding 200° C., polyvinyl alcohol was seen to undergotransmutation.

Thus, it may be seen from FIG. 17 that the proton conductor film needsto be heated at a temperature of from about 150° C. to about 200° C.,and that preferably the proton conductor film is to be heated at atemperature from about 160° C. to about 200° C.

EXAMPLE 2

Then, fullerenol in the form of a hydrogen sulfate ester and polyvinylalcohol were mixed together at a weight ratio of 2:1 and dispersed ordissolved in a quantity of water equal to five times by weight as muchas the total weight of fullerenol in the form of a hydrogen sulfateester and polyvinyl alcohol to prepare a slurry. This slurry was thencoated on the layer of fullerenol in the form of a hydrogen sulfateester, using a mask and a squeezee. Then, water was vaporized off toform a proton conductor film, 12 μm in thickness, containing fullerenolin the form of a hydrogen sulfate ester and polyvinyl alcohol.

A liquid mixture was prepared by dispersing or dissolving a mixture offullerenol in the form of a hydrogen sulfate ester and polyvinyl alcoholin a weight ratio of 2:1 in a quantity of water equal to five times asmuch as the total weight of fullerenol in the form of a hydrogen sulfateester and polyvinyl alcohol. This liquid mixture was coated on onesurface of the so formed proton conductor film by a gravure printingmethod and dried to vaporize water off to form a layer of the mixture offullerenol in the form of a hydrogen sulfate ester and polyvinyl alcoholto a thickness of 1 μm.

A heater was directly applied, under pressure, to the so producedlaminated proton conductor to effect heating at 180° C. for ten seconds.

A hydrogen gas then was supplied, at a pressure of 0.03 MPa, from theside of the laminated proton conductor not carrying the layer of themixture of the fullerenol in the form of a hydrogen sulfate ester andpolyvinyl alcohol, and measurements were made of temporal changes in thehydrogen gas concentration on the side of the laminated proton conductorcarrying the layer of the mixture of the fullerenol in the form of ahydrogen sulfate ester and polyvinyl alcohol.

The measured results are shown in FIG. 18.

COMPARATIVE EXAMPLE

In the same way as in Example 2, a hydrogen gas was supplied at apressure of 0.03 MPa from one side of the laminated proton conductor andmeasurements were made of temporal changes on the opposite side of thehydrogen gas concentration.

The measured results are shown in FIG. 18.

It may be seen from FIG. 18 that the proton conductor film of Example 2,carrying the layer of the mixture of the fullerenol in the form of ahydrogen sulfate ester and polyvinyl alcohol on its one surface exhibitshydrogen gas intercepting performance which is superior to that of theproton conductor film of Comparative Example not carrying the layer ofthe mixture of the fullerenol in the form of a hydrogen sulfate esterand polyvinyl alcohol.

A hydrogen gas then was supplied, at a pressure of 0.03 MPa, from theside of the proton conductor film of Example 2 carrying the layer of themixture of the fullerenol in the form of a hydrogen sulfate ester andpolyvinyl alcohol, and measurements were made of temporal changes in thehydrogen gas concentration on the side of the proton conductor filmcarrying the layer of the mixture of the fullerenol in the form of ahydrogen sulfate ester and polyvinyl alcohol. No significant differencewas noticed.

The present invention is not to be limited to the embodiments, as nowexplained, but may be suitably modified without departing from the scopeof the invention as defined in the claims.

The proton conductor manufactured by the process of the presentinvention in an embodiment includes a fullerene derivative including aproton dissociative group introduced to at least one carbon atom of thefullerene molecule, and a polyvinyl alcohol in an amount of more thanabout 20 wt %, and hence demonstrates high protonic conductivity even ina dry state in a temperature range inclusive of the room temperature.Moreover, proton conductor exhibits film-forming properties, ascribableto polyvinyl alcohol amount, and hence is improved in strength, whileprohibiting gas transmission. Thus, the proton conductor can be used asa thin film exhibiting high protonic conductivity.

Moreover, the proton conductor can be used for an electro-chemicaldevice without being atmosphere limited so that it is possible torealize the small size and simplified structure of the system.

According to the present invention, there may be provided a protonconductor film for a fuel cell having a high hydrogen gas interceptingperformance without the risk of lowering the cell output, and a fuelcell which is able to realize a high fuel output.

By employing this proton conductor for an electro-chemical device, thereis imposed no atmosphere constraint and hence the system can be reducedin size and simplified in structure.

The present invention moreover renders it possible to provide a protonconductor film in which there is no risk of dissolution even thoughwater is yielded by electrode reaction and which exhibits superiorhydrogen gas intercepting performance, a method for preparation of theproton conductor film, a high output fuel cell having the protonconductor film in which there is no risk of dissolution even thoughwater is yielded by electrode reaction and which exhibits superiorhydrogen gas intercepting performance, and a method for preparation ofthe fuel cell.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its intended advantages. It is therefore intendedthat such changes and modifications be covered by the appended claims.

1. A proton conductor comprising: a fullerene derivative including afullerene molecule and a proton dissociative group introduced to atleast one carbon atom of the fullerene molecule; and a polyvinyl alcoholin an amount of more than about 20 wt %.
 2. The proton conductoraccording to claim 1 wherein the amount of the polyvinyl alcohol isabout 40 wt % or less.
 3. The proton conductor according to claim 1wherein the proton dissociative groups are —XH, where X is a bivalentatom or atom group thereof and wherein H is a hydrogen atom.
 4. Theproton conductor according to claim 1 wherein the proton dissociativegroups are —OH or —YOH, where Y is a bivalent atom or atom group thereofand wherein H is a hydrogen atom.
 5. The proton conductor according toclaim 4 wherein the proton dissociative groups are selected from thegroup consisting of —OH, —OSO₃H, —COOH, —SO₃H, —OPO(OH)₂ andcombinations thereof.
 6. The proton conductor according to claim 1wherein one or more proton dissociative groups and electrophilic groupsare introduced into the fullerene molecules.
 7. The proton conductoraccording to claim 6 wherein the electrophilic groups at least include anitro group, a carbonyl group, a carboxylic group, a nitrile group, ahalogenated alkyl group, a halogen atom and combinations thereof.
 8. Theproton conductor according to claim 1 wherein the fullerene moleculeincludes spheroidal carbon cluster molecules that are represented by theformula C_(m), where m is a natural number such that the fullerenemolecules are configured in a spherical structure.
 9. The protonconductor according to claim 1 wherein the proton conductor is formed toa thin film having a thickness of about 300 μm or less.
 10. A method forproducing a proton conductor comprising: introducing proton dissociativegroups to carbon atoms of fullerene molecules to produce a fullerenederivative; mixing the fullerene derivative with about 20 wt % or moreof a polyvinyl alcohol; and forming the fullerene derivative and thepolyvinyl alcohol mixture into a thin film.
 11. The method according toclaim 10 wherein the polyvinyl alcohol is mixed in an amount of about 40wt % or less.
 12. The method according to claim 10 wherein the protondissociative groups are —XH, where X is a bivalent atom or atom groupthereof and wherein H is a hydrogen atom.
 13. The method according toclaim 10 wherein the proton dissociative groups are selected from thegroup consisting of —OH, —YOH, and combination thereof where Y is abivalent atom or atom group thereof and wherein H is a hydrogen atom.14. The method according to claim 13 wherein the proton dissociativegroups are selected from the group consisting of —OH, —OSO₃H, —COOH,—SO₃H, —OPO(OH)₂ and combinations thereof.
 15. The method according toclaim 10 wherein one or more proton dissociative groups andelectrophilic groups are introduced into the fullerene molecules. 16.The method according to claim 15 wherein the electrophilic groups areselected from the group consisting of a nitro group, a carbonyl group, acarboxylic group, a nitrile group, a halogenated alkyl group, a halogenatom and combinations thereof.
 17. The method according to claim 10wherein the fullerene molecules are spheroidal carbon cluster moleculesrepresented by C_(m), where m is a natural number with which thefullerene molecules can constitute a spherical structure.
 18. The methodaccording to claim 10 wherein the proton conductor is formed into a thinfilm having a thickness of about 300 μm or less.
 19. An electro-chemicaldevice comprising: a first electrode; a second electrode; and a protonconductor between the first electrode and the second electrode; whereinthe proton conductor comprises a fullerene derivative including afullerene molecule and a proton dissociative group introduced to atleast one carbon atom of the fullerene molecule, and polyvinyl alcoholin an amount of more than about 20 wt %.
 20. The electro-chemical deviceaccording to claim 19 wherein the amount of polyvinyl alcohol is about40 wt % or less.
 21. The electro-chemical device according to claim 19wherein the proton dissociative groups are —XH, where X is a bivalentatom or atom group thereof and wherein H is a hydrogen atom.
 22. Theelectro-chemical device according to claim 19 wherein the protondissociative groups are selected from the group consisting of —OH, —YOHand combinations thereof where Y is a bivalent atom or atom groupthereof and wherein H is a hydrogen atom.
 23. The electro-chemicaldevice according to claim 22 wherein the proton dissociative groups areselected from the group consisting of —OH, —OSO₃H, —COOH, —SO₃H,—OPO(OH)₂ and combinations thereof.
 24. The electro-chemical deviceaccording to claim 19 wherein one or more proton dissociative groups andelectrophilic groups are introduced into the fullerene molecules. 25.The electro-chemical device according to claim 24 wherein theelectrophilic groups at least include a nitro group, a carbonyl group, acarboxylic group, a nitrile group, a halogenated alkyl group, a halogenatom and combinations thereof.
 26. The electro-chemical device accordingto claim 19 wherein the fullerene molecules are spheroidal carboncluster molecules represented by C_(m), where m is a natural numberallowing the fullerene molecules to constitute a spherical structure.27. The electro-chemical device according to claim 19 wherein the protonconductor is formed into a thin film having a thickness of about 300 μmor less.
 28. The electro-chemical device according to claim 19 whereinthe device is configured as a fuel cell.
 29. The electro-chemical deviceaccording to claim 19 wherein the device is configured as a hydrogen-aircell.
 30. The electro-chemical device according to claim 19 wherein atleast one of the first and second electrodes is a gas electrode.
 31. Theelectro-chemical device according to claim 19 wherein at least one ofthe first and second electrodes is an active material electrode.